Your guide to buying a DSLR lens

Yesterday I uploaded a new page (Submit your Q!) where you guys can go and ask me anything that’s been on your mind regarding photography, and I would dedicate a blog post explaining the matter in detail. Our first question came from Guneet Narula from India, who has bought a Canon 1000D EOS DSLR with its kit lens. Guneet is going on trip to the Himalayan Mountains very soon, and he is looking into getting a second lens to take with him, and was wondering which kind of lens would be of more use during his vacation?

Thank you very much Guneet for submitting your question. This is an excellent question and in order to let you in on what kind of lens you might wanna take with you, I’m going to talk a little bit about lenses first, the focal length, angle of view and the crop factor. And in the end, I’m going to suggest a few types of lenses you might want to consider for your trip depending on the type of photos you are most likely to be taking.

What is a lens focal length?

When light travels through the lens hitting the digital sensor or photographic film, different light rays converge onto a point called the focal point. The focal point of any given lens is the distance between that convergence point and the lens in mm. Lenses with shorter focal lengths are called short lenses and they have a wide angle of view. Lenses with longer focal lengths are called long lenses and they have a narrower angle of view.

Lenses that are roughly similar to the human visual system’s angle of view are called normal lenses. These are of a focal length something between 50-60mm. Lenses that are shorter are called wide angle or short lenses. And, lenses that are longer than that are called long or telephoto lenses.

There are two types of lenses: there are prime lenses which have a fixed focal length, and there are zoom lenses which have a varying focal length within a given range.

When you’re out shopping for a new lens, there are a few factors in question which I have actually previously covered in an earlier series of posts:

1. Shopping for a new lens: Image Quality
2. Shopping for a new lens: Lens Aperture
3. Shopping for a new lens: Lens Minimum Focusing Distance
4. Shopping for a new lens: Image Stabilization

Crop factor (focal length multiplier)

Another important aspect to understand with regards to lenses and cameras is the crop factor. The crop factor, also known as the focal length multiplier (FLM), is the ratio of the dimensions of a camera’s image plane compared to a reference format. Most often, this term is applied to digital cameras, relative to 35mm film format as a reference. In the case of digital cameras, the image plane would be a digital sensor.

Lenses focal lengths are given relative to a 35mm film, which is called a full frame. Some digital cameras have a smaller sensor size than that of a full frame, so a lens’ focal length will actually vary and change to a different focal length depending on the sensor’s size, or what is known as the sensor’s crop factor.

The angle of view of a given lens will depend on the camera’s sensor size as well. A smaller sensor size will virtually further elongate the focal length of a given lens and tighten the angle of view. A full frame, or a 35mm sensor size, preserves the suggested angle of view of any given lens.

This means that, a 50mm lens on a smaller sensor size will have a larger focal length and consequently a smaller field of view than that of a full sensor size. The same lens will seem to magnify the scene, when in fact the lens is actually just cropping the scene giving a result of a zoomed in view.

Taking that into account, let’s assume you’re shooting with a 1.6x crop factor image plane at 50mm focal length. Your actual focal length would then be 50 X 1.6 = 80mm rather than 50mm.

On a full frame (35mm) digital camera sensor, the crop factor is 1. So the focal length of a given lens is the actual focal length you will get on such a full frame sensor.

From a practical point of view, a focal length of any given lens can be thought of as the degree of magnification that the lens provides. Longer focal lengths seem to further magnify the scene. Lens focal length also influences the perspective and the compression level of the scene.

Telephoto lenses

Telephoto lenses appear to magnify the photographed subject. They give you the ability of getting a clear view of a distant subject, without you having to move closer to frame your shot.

Telephoto lenses are commonly used for sports and nature photography where the distance a photographer can approach their subject is limited due to safety or flexibility reasons.

Telephoto lenses also provide a shallow depth of field. The longer the lens, combined with larger lens openings provide shallower depths of field. Smaller lens openings combined with long focal lengths make elements within the frame seem closer or more compressed.

Wide angle lenses

Wide angle lenses are lenses with shorter focal lengths (typically shorter than 50mm lenses on a full frame sensor). They provide a wide angle of view of the scene and give the ability for the photographer to move in tight and close to their photographed subject.

Wide angle lenses are commonly used in landscape photography, and they provide greater depths of field especially with smaller lens openings. They tend to accommodate more of the scene within the frame and this is why they are sometimes used by street photographers and photojournalists to give a feeling of the atmosphere of the setting and draw the viewer into the image.

Wide angle lenses can also be used by portrait and fashion photographers that like to include more of the scene along with their photographed subject or model.

Camera shake

To calculate the slowest shutter speed that is possible for a hand held camera with no noticeable blurring of the image, it needs to be 1/lens focal length you’re shooting with.

This means that if you’re shooting with a 50mm lens, your minimum shutter speed should be no longer than 1/50, for example 1/50, 1/60, 1/125, 1/250…etc to avoid image blur.

Likewise, if you’re shooting with a 70-200mm zoom lens at let’s say 125mm focal length, your minimum shutter speed should be no longer 1/125, for example 1/125, 1/250, 1/500… etc. But if your lens offers image stabilization, you would be able to take the slowest shutter speed possible down an extra 2-4 stops or even more. This means you would be able to shoot at 1/15 or 1/8 or even slower, with no significant image blurring. This is of course assuming you have a 35mm format camera.

If you’re not using a 35mm format, you need to note that the crop factor will affect the slowest shutter speed possible at a given focal length.

So, what kind of lens might Guneet wanna get for his trip?

Well Guneet, you already have a kit lens. That is a 18-55mm f/3.5-5.6 IS Lens for the Canon 1000D EOS DSLR. Keeping in mind that the Canon 1000D has a crop factor of 1.6x, the actual focal length your kit lens provides is 28-88mm (18-55mm x 1.6).

I say, you already have a short lens with a wide angle of view when your kit lens is zoomed all the way out to 18mm. You also have a somewhat medium-range to short telephoto when the lens is zoomed all the way in to 55mm.

Now, depending on what type of photographs you might be taking, a shorter lens (something that is less than 18mm since you already have an 18mm lens) will give you the ability to shoot wider and sharper landscapes. If you ask me, you don’t really need to get a short lens cause your current lens is short enough for landscape work. So unless you are after a better quality or a smaller minimum aperture value, the lens you already have will suffice.

If, however, you’re looking to shoot wide life, then a telephoto lens (something in the range of 100-300mm for example would be great). If you are just looking to shooting general portraits, then your current lens is more than enough (though you might wanna consider the cheap, highly-praised 50mm f/1.8 lens). The small aperture enables you to handhold your camera even under lower lighting conditions with no significant camera shake.

You did mention you were going to take your tripod with you, so if you are looking to shoot with a telephoto lens, be sure you mount your camera on your tripod. With a short lens, you can pretty much get away with hand holding your camera during faster shutter speeds.

Some lens options for you would be:

• Canon EF-S 18-200mm f/3.5-5.6 IS Standard Zoom Lens
• Canon EF-S 55-250mm f/4.0-5.6 IS Telephoto Zoom Lens
• Canon EF 75-300mm f/4-5.6 III Telephoto Zoom Lens
• Canon EF 50mm f/1.8 II lens

So Guneet thank you very much for sending in your question. I hope this helps you better decide what kind of lens you would wanna go with. We hope you have a wonderful trip, and take lots and lots of amazing photos. Be sure to share your images with us when you get back =) good luck my friend.

So guys, if anyone has a photography question they’d like to ask, please head over to the Submit your Q! page and post your question right now!